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Nucleus

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Objective: To find and learn about the nucleus!

Argh it’s crowded again! But here’s good news and bad news! Good news: The nucleus is one of the most visible organelles in the cell, so we should be able to find it easily. The bad news: That’s the only hint I have. But if you insist on finding the nucleus so badly, read on for more information about the brain of the cell!

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The Nucleus:

Genetic Information:

The organelle that contains the genetic code of the cell is called the nucleus, and it contains DNA (deoxyribose nucleic acid), which is made out of chromosomes. These chromosomes, which made of chromatins, are units of DNA that have genetic information vital to keep the cell’s functions under control. We can usually spot chromosomes when a cell is in mitosis or meiosis, when the chromatin fibers condense into distinct shapes called chromosomes. Otherwise, the chromatids are usually in a jumbled mess inside the nucleus! Each eukaryotic species has a distinct number of chromosomes that help us identify what it is.

Chromosomes are composed of chromatin, which is made of DNA wrapped around proteins called Histones.

Nuclear Envelope: One interesting thing about the nucleus is that it has a nuclear envelope with two phospholipid bilayer membranes, which help to maintain a different environment inside the nucleus from the cytoplasm. This envelope has many protein structures called pore complexes distributed along the membrane to regulate exchange of proteins, RNA, and large macromolecules and keep the environment inside the nucleus at homeostasis. The nucleus not only has it’s own fancy double membrane, but it also has it’s own structural device called the nuclear lamina, made up of a network of protein filaments, that physically supports the nuclear envelope. This structure lines the nuclear surface side of the envelope. Another structural complex is called the nuclear matrix, which is also an assortment of fibers that act like the cell’s cytoskeleton to further support the nucleus’ shape.

This simple diagram labels the major components of the nuclear envelope. Note that it has two membranes: the inner nuclear membrane (INM) and the outer nuclear membrane (ONM)

Nucleolus: Now that we’ve gone past the tedious but important nuclear envelope, let’s get onto the nucleolus! From an outside view it really looks like another nucleus inside a nucleus, but don’t be tempted to go with such a simple and absurd explanation. It’s complicated, but it’s all really easy to understand! So basically the nucleolus has proteins and genetic information that help out with the synthesis of RNA (ribose nucleic acid). More specifically, ribosomal RNA (rRNA) is made in a process called transcription, during which the DNA double helix unzips and proteins help to transcribe, or copy, the correct sequence of nucleotides onto the rRNA from the DNA. In other words, this is where the cell copies its DNA instructions onto rRNA to make proteins later on in ribosomes by sending the messenger RNA (mRNA) to do translation. In addition, the nucleolus makes small ribosomal subunits out of rRNA and proteins (from the cytoplasm), which are then sent out to the cytoplasm to combine with another larger subunit to form a ribosome. In short, the nucleolus is important for RNA and ribosomes.

RNA transcription/synthesis occurs in the nucleolus. RNA is used for copying the DNA sequence and ultimately constructing proteins, so it is vital that RNA synthesis goes well.

Extra stuff:

Here are two videos from the NDSU Virtual Cells Animations Project that describe transcription and translation: (remember, transcription in nucleolus, translation in free ribosomes floating around in the cytoplasm!)

TranscriptionTranslation

Sumanas, Inc. also provides an animated video about the translation of RNA.